This invention relates generally to optical switches employed in communications and other applications for routing beams of optical radiation and, more particularly, to an optical switch employing electro-optic material in the form of a retarder and dielectric polarizing beam splitter films in such a manner as to provide a switch having low crosstalk characteristics.
An elementary building block for optical switching consists of the fully symmetrical 2.times.2 switch. The switch includes four ports through which light may enter or exit, and it exhibits two distinct states. Each state connects the four ports in two disjoint pairs. Thus, for example, in one state ports one and four are connected by an optical path while simultaneously ports two and three are connected by another optical path. In the other state, ports one and two are connected by an optical path, while ports three and four are connected by still another optical path. Such a switch is fully symmetrical in that the optical paths are fully bi-directional, and the ports are indistinguishable. Aside from these topological considerations is the question of what type of light can be switched. The most general switch design can operate with unpolarized light beams. Other switch designs are only capable of switching light having a particular state of polarization, usually linearly polarized. Such 2.times.2 switches have a wide variety of applications ranging from simple on-off switching to the combination of many such switches into switching arrays or crossbars. There is a need, in particular, for such switches in the field of fiber optic communications where it is necessary to switch unpolarized light.
Three characterisitcs of 2.times.2 switches are of particular importance in constructing practical optical switches. These are 1) the amount of time required to switch from one state to the other, i.e. the switching time; 2) the fraction of the input light that exits from the wrong or unselected port, i.e. the crosstalk; and 3) the fraction of the input light that fails to exit from the selected port, i.e. the insertion loss. Different practical switch designs differ as to the values of these characteristics which thus limit the range of their potential applications. There are also applications which do not require the full symmetry of the 2.times.2 switch, and practical switch designs can thus sacrifice some aspect of the full symmetry to achieve enhanced values for one or more of these three characteristics.
The original electro-optic switch related to the present invention is described by R. E. Wagner and J. Cheng, Appl. Opt. 19, 2921 (1980). This teaching was the first to show how to use bulk optics in the form of passive polarizing beam splitters made with dielectric films together with an electro-optic retarder comprising a nematic liquid crystal film to make a 2.times.2 switch capable of switching nearly all of the light among multimode optical fibers. Switches based on this prior art design generally switch in 10 milliseconds and exhibit, at best, -20 dB of crosstalk. The insertion loss of this design depends on the source of the input beam, being somewhat better than -1 dB for a well collimated laser beam or as much as a few dB for light from an optical fiber that has been collimated with a gradient index rod lens.
The later prior art teachings of R. A. Soref, Opt. Lett. 6, 275 (1981) illustrated the way in which a compound switch exhibiting a low crosstalk characteristic can be formed by cascading four simple switches having a higher crosstalk characteristic. Thus, cascading simple switches having a -20 dB crosstalk characteristic can produce a compound switch exhibiting a -40 dB crosstalk characteristic. The number and cost of the parts required for constructing such a compound switch are, however, both high. The crosstalk was -27 dB for Soref's prototype switch, and the switching time remained about 10 milliseconds, since he employed a nematic liquid crystal retarder. The insertion loss was large, in part because the more complicated switch possessed a greater number of reflection losses. Still later, the teachings of R. A. Soref and D. H. McMahon, Opt. Lett. 7, 186 (1982) described a low crosstalk switch in which the polarizing beam splitter films were replaced with a configuration of three calcite prisms. Although the crosstalk characteristic of this prior art switch improved to -32 dB for switching laser beams, the switch was not fully symmetric, and the considerable cost of the calcite made it very expensive. The switching time of this switch was also about 10 milliseconds, while the insertion loss was about -3 dB. Still later, U.S. Pat. No. 4,948,229 to Soref taught another 2.times.2 switch that employs a fast-switching ferroelectric liquid crystal retarder, and its crosstalk characteristic can be shown to be in the -20 dB range.
Despite these prior art attempts to construct an improved optical switch, there remains today a need for a 2.times.2 optical switch that uses a small number of inexpensive parts and that exhibits a crosstalk characteristic better than -20 dB.
It is therefore a principal object of the present invention to provide such an optical switch that may be constructed from a small number of inexpensive parts and that exhibits a low crosstalk characteristic. This and other objects are accomplished in accordance with the illustrated preferred embodiments of the present invention by arranging electro-optic retarders and polarizing beam splitters in novel configurations.